The present invention relates generally to circuits for driving inductive loads and, more particularly, to a method and apparatus for driving inductive loads wherein stresses induced on switching devices used in drive circuits are reduced with resultant improvements in device reliability, system electrical noise and current sensing relative to the loads. While the present invention is generally applicable to inductive load drive circuits, it is particularly applicable to pulse width modulated (PWM) inverter systems for driving alternating current (AC) motors and accordingly will be described with reference to that application.
For operation of an AC motor from a direct current (DC) power source, a pulse width modulated (PWM) inverter system may be used to convert the DC power to AC power for operating the motor. An AC waveform is constructed by the inverter by alternately connecting a positive power bus and a negative power bus to the motor in accordance with prescribed PWM signals to construct a desired AC waveform. See, for example, U.S. Pat. No. 4,825,132 which issued to Gritter and illustrates a three-phase pulse width modulated inverter circuit used for motor control. As illustrated in the, Gritter patent, the inverter circuit includes discrete free-wheeling diodes which are connected in back-to-back relationship with inverter transistors to maintain current flow in the motor whenever an inverter transistor is turned off. While discrete free-wheeling diodes are required for bipolar inverter transistors, if MOSFET devices are used as the inverter transistors, free-wheeling body diodes are inherently included within the structure of the MOSFET devices.
A problem arises, especially when PWM is used, if the free-wheeling diodes have a long reverse recovery time, which is characteristic of the body diodes inherent in MOSFET devices. The long reverse recovery time of the free-wheeling diodes results in large transient current peaks which can be a source of electrical noise. Electron irradiation of power MOSFET devices has been used to improve the characteristics of the body diodes. However, even with such irradiation, the reverse recovery effects of the body diodes are substantial and remain as a problem. Further, the voltage applied to the motor changes levels very rapidly which can also be a possible source of electrical noise.
It is possible to at least partially overcome the long reverse recovery times of the MOSFET body diodes by using Schottky diodes in series with MOSFET power transistors and discrete fast recovery diodes connected as shown in Gritter. Unfortunately, such arrangements add two additional components to each MOSFET power transistor, which components must be sized and packaged to handle substantial power levels. Further, the Schottky diodes add to the power loses in the circuits and these additional losses are substantial particularly at high current levels commonly encountered in power inverter circuits.
Power loss problems are also created by the long recovery times typical of inherent MOSFET body diodes when used in Class-D audio power amplifiers as described in U.S. Pat. No. 4,626,715 issued to Ishii. In the Ishii patent, the power loss problems are overcome by means of inductors which are connected between the outputs of the amplifier drive transistors and the load, and diodes which are connected between the load and the power sources for the amplifier. The inductors of the power amplifier circuit of Ishii are operated in a non-saturating mode over the operating range of the amplifier and are noted as being sufficiently small that ordinary circuit operation is not affected. While such inductors are satisfactory for use in Class-D audio power amplifiers, their use in a power inverter circuit such as used to drive AC motors is unacceptable.
Accordingly, the problems associated with the reverse recovery times of free-wheeling diodes used in inverter circuits, and particularly the long reverse recovery times characteristic of inherent body diodes of MOSFET power transistors used as the free-wheeling diodes in inverter circuits which include such transistors, remain.
A need thus exists for reducing the stresses induced on switching devices of power inverter circuits which drive inductive loads during reverse recovery of free-wheeling diodes of the power inverter circuits and the concurrent electrical noise and current sensing problems.